The invention concerns a fishfarming cage for use in water. An increasing part of the world's food production is expected to come from aquaculture or farming of aquatic organisms, as the requirement for food increases and the stocks available for fisheries or catching decreases. Aquaculture in artificial or natural basins and pools on shore is well known. A rice field with carps is an example on a biotope where one organism, i.e. rice, benefits from excrements from fish. Fishfarming cages (fish cages or cages) and fish farms have for many years also been localized in the sea, in lakes and in rivers to save on real estate costs and to benefit from a natural supply of fresh water.
Conventional fish cages can comprise a fish net suspended from floating elements. Such fish cages are easily torn by collisions with vessels or large objects floating in the currents. The load imposed on the net from the ambient environment, e.g. waves or powerful currents, can also cause tearing of the net.
Tear damages gives a substantial risk for escape. Risk for escape can also increase because some species, for instance cod, gnaw on the nets. Animals escaping from fish farms represent an economic loss and a genetic and ecological risk.
Other problems with conventional open fish net cages are that pollution, algae, infectious matter and parasites easily enters the farm from the surrounding water, and that the open farms themselves are a substantial source of pollution because excrements, unconsumed feed, unused medicaments and other effluents are led by the current to the environment around the farm. Roe and milt from an open farm can also cause genetic pollution. When many animals live in a small area in an open cage, the oxygen level may decrease in the surrounding water. This can increase the growth of algae, which in turn can create new problems for an unprotected open conventional fish farm.
The availability of suitable locations for conventional open farms with flowing, oxygen-rich water having limited wave height is limited from the outset. One problem is that the availability of such places is reduced because they are harmed or damaged by pollution from fish farms, or because the authorities do not permit localizing of fish farms in environments where the pollution may harm the environment.
Cleaning is also a problem with conventional cages. Under normal operation, cleaning of nets comprise a substantial part of the operational costs. Additionally, long quarantine periods are required after outbreak of disease in order to avoid carrying over the disease to new livestock in the cage. Hence, it may be advantageous if the cage can be adapted to different species, such that diseases and parasites depending on species, e.g. salmon, dies away while other species, e.g. halibut, scallops or clams, which are not hosts to salmon disease or parasites, are produced in the cage.
Streaming water is important to make a number of pelagic species form schools and thereby prevent aggressive behaviour which otherwise might limit the maximum number of fish in the cage. Bottom-dwelling species depending on feed in a passing flow, e.g. a tidal current, obviously also need flowing water to thrive. The availability of locations having natural flowing water is limited, and will likely further decrease with expected future environmental regulations. The need for robust closed fish cages with controlled internal flow conditions is expected to increase as the availability of convenient, protected places with suitable flow conditions is reduced. It will also be an advantage if the flow in such a cage can be reversed at regular intervals, because some species, e.g. salmon, becomes blind on one eye if they swim in the same direction continuously, and because some species living in tidal currents can benefit if the current alternates at regular intervals.
The lighting conditions in the fish cage can also be important, e.g. in fish farming. Early maturation of commercially important species as salmon and cod is a problem that can be reduced substantially by using light. When the fish produces roe or milt, the harvest weight is reduced and there is a risk that the fish must be fed for another 12-18 months before it regains a similar harvest weight.
U.S. Pat. No. 4,798,168 discloses an enclosure for farming fish, shell fish and other marine beings solving some of the problems above. A flexible bag of preferably watertight cloth is used instead of an open net, and prevents pollution, algae, parasites and infectious matter from entering the enclosure. An outlet for unconsumed feed, excrements and other pollution prevents or reduces discharge of effluent to the surrounding waters. Desired flow is ensured by water inlets-expelling water in or near the surface perpendicular to the cage's radius to cause rotation of the water within the cage. Air and/or oxygen is added to the water before it is supplied to the cage. The lighting conditions can be controlled by providing a roof or cover made from a light impervious cloth over the bag, and by making the flexible cloth comprising the bag translucent. The costs of cleaning the farm can be reduced by threading a fresh bag outside the old one, and then remove the used bag. The main problems of this farm is that the flexible bag is subject to tear damage, that the water inlet provides too high water velocities close to the perimeter and/or too low water velocities closer to the centre, and that the cage is suitable for certain types of fish only. Another problem with the bag is that the water level within the bag must be kept higher that the water level outside it in order to maintain a substantial circular form when seen from above.
NO 169 751 B, NO 160 753, GB 1 594 832, NO 150 741, U.S. Pat. No. 3,870,018 and NO 175 341 show examples on fish farming basins and cages having rigid walls and an outlet for excrements and other effluent in the bottom. All of them have a substantially cylindrical upper part, a conical lower part, and rotating water within as illustrated by FIG. 1b. The purpose of the conical part is to increase the tangential speed of the water near the bottom, and thus improve the drainage. A problem with this shape is that the straight walls give resistance in the water to currents and waves, and thereby pushes on the water within the cage. Hence, wave movements are easily transmitted to the water within the cage, and will increase the risk for swashing (uncontrolled exchange of water) from inside or outside the cage. This increases the risk for escape and biological contamination. Hence, such fish farming vessels are unsuitable for use in places with waves that can cause such swashing. An objective of the present invention is therefore to provide a floating fish cage which is not torn apart in waters with waves and currents, and which at the same time easily can be adapted to different species' requirements for flow conditions, lighting conditions, water quality etc.